The pyrolysis of 1,1,1-trifluoroethane (viz., methylfluoroform) at elevated temperatures (about 400.degree. C. to about 1500.degree. C.) to form 1,1-difluoroethene (viz., vinylidene fluoride) and hydrogen fluoride is known; see U.S. Pat. Nos. 2,480,560; 3,188,356; and 3,456,025, the disclosures of which are, in their entireties, incorporated herein by reference. The reaction is reversible and may be represented by the equation EQU CH.sub.3 CF.sub.3 .revreaction.CH.sub.2 .dbd.CF.sub.2 +HF
At the elevated temperatures of the pyrolysis, the gas-phase reaction proceeds readily to the right, while in the liquid phase the reaction proceeds rapidly to the left.
Two experiments were conducted which illustrate the rapidity of the reverse reaction in the liquid phase. Individual sources of nitrogen, hydrogen fluoride, and 1,1-difluoroethene were connected through valves, tees and rotameters to the entrance tube of a reaction vessel. The reaction vessel was a Kel-F.RTM. polymer test tube having a length of 15.24 centimeters and a diameter of 3.81 centimeters equipped with a neoprene two-hole stopper and containing a magnetic stirring bar. The entrance tube passed through one hole of the stopper and was equipped with a polyethylene frit for dispersal of the introduced gas. Sufficient clearance was allowed between the frit and the bottom of the reactor for the magnetic stirring bar to spin. An exit tube passed through the second hole of the stopper and permitted gas to be removed from the upper portion of the reaction vessel. The exit tube was sequentially connected to a water scrubber, a drying tube containing potassium hydroxide pellets, and an exit rotameter. Gas samples were taken through rubber tubing connected to the outlet of the exit rotameter. The rotameter for measuring the introduced 1,1-difluoroethene and the exit rotameter were identical rotameters. This permitted a comparison of the 1,1-difluoroethene entrance flow rate and the exit gas flow rate during the experiments. The reaction vessel was immersed in a cooling bath positioned over a magnetic stirrer drive unit. For the experiment conducted at 0.degree. C., wet ice was used as the cooling bath. For the experiment conducted at -25.degree. C., a solid carbon dioxide and carbon tetrachloride bath was used. At the start of each experiment, nitrogen was passed through the system and the reaction vessel was cooled to the desired temperature. The flow of nitrogen was discontinued and hydrogen fluoride was condensed to provide 75 milliliters of liquid in the reaction vessel. The flow of hydrogen fluoride was then discontinued and 1,1-difluoroethene was bubbled at a flow rate of 40 milliliters (referenced to 25.degree. C. and ambient atmospheric pressure) per minute into the liquid hydrogen fluoride. In both cases there was substantial absorption of 1,1-difluoroethene in the hydrogen fluoride during approximately the first five minutes of flow. Thereafter, the entrance and exit flow rates were identical. Gas-liquid chromatographic analyses of samples of the exit gas taken after establishment of identical flow rates showed that 95 percent of the introduced 1,1-difluoroethene reacted with hydrogen fluoride at 0.degree. C. to form 1,1,1-trifluoroethane, while 96 percent of the introduced 1,1-difluoroethene reacted with hydrogen fluoride at -25.degree. C. to form 1,1,1-trifluoroethane.
Because of the rapidity of the reverse reaction at low 1,1-difluoroethene, it was long believed that partial condensation of the reaction products of the pyrolysis reaction to achieve the recovery of 1,1-difluoroethene which is essentially free of hydrogen fluoride would not be practical in a commercial process. The prior art processes, therefore, sought to quickly convert the hydrogen fluoride to a form which would be essentially unreactive with 1,1-difluoroethene by the time low temperatures favoring reversion were reached.
In Examples I and II of the U.S. Pat. No. 2,480,560 the pyrolysis reaction products were washed with water, presumably in a quenching operation. The ultimate yields of 1,1-difluoroethene are not given, but in any event the hydrogen fluoride would be absorbed by the water to form aqueous hydrofluoric acid. Substantially anhydrous hydrogen fluoride can be obtained from aqueous hydrofluoric acid, but because of the high affinity of hydrogen fluoride for water, the dehydration processes are energy intensive and both capital expenditures and operating costs are high.
U.S. Pat. No. 3,456,025 discloses the removal of hydrogen fluoride with water or an aqueous solution of caustic; in the Example aqueous caustic was used. The reaction of hydrogen fluoride with aqueous caustic produces sodium fluoride from which hydrogen fluoride can be obtained by acidification with a mineral acid such as sulfuric acid. This regeneration process requires water removal at some point in the process. It is also ultimately energy intensive, expensive from the standpoints of capital expenditures and operating costs, and produces a by-product such as sodium sulfate that must be used in some fashion or properly disposed.
In the Examples of U.S. Pat. No. 3,188,356, the pyrolysis reaction products were passed through a tube packed with sodium fluoride heated to 100.degree. C. to remove hydrogen fluoride. Judging from J. F. Froning et al, "Purification and Compression of Fluorine", Industrial and Engineering Chemistry, March 1947, pages 275-278, and J. H. Simons, Fluorine Chemistry, Vol. I, Academic Press, Inc., New York, 1950, pages 310-311, both of which deal with the removal of hydrogen fluoride from elemental fluorine, the removal of hydrogen fluoride according to U.S. Pat. No. 3,188,356 would be accomplished through formation of the sodium fluoride-hydrogen fluoride complex, NaF.HF. Both of these documents present a table of equilibrium pressures of hydrogen fluoride over a mixture of NaF and NaF.HF at various temperatures and the Froning et al paper discusses operating an absorption tower containing sodium fluoride to alternately absorb hydrogen fluoride from elemental fluorine and then to regenerate hydrogen fluoride. It would accordingly be expected that such a cyclic absorption-regeneration technique would be effective in removing hydrogen fluoride from the pyrolysis products of U.S. Pat. No. 3,188,356 and in providing substantially anhydrous hydrogen fluoride. In order to incorporate such cyclic absorption-regeneration techniques into a continuous system, however, a plurality of units operating at differing phases in the cycle must be employed. The capital expenditures are therefore high. Inasmuch as the absorption bed must be heated and cooled in cyclic fashion, the energy requirements and operating expenses are high. It is more efficient to avoid the incorporation of cyclically functioning units in an overall continuous process when this is possible.